Safety level of transport vehicles has been increasing every year, and it is essential to protect occupants in a cabin even if the function of the transport vehicle is damaged in collision. Therefore, in order to make a frame around the cabin absorb the energy that is generated in collision and reduce the shock transmitted to the cabin, a high strength steel sheet is actively used for the frame, whereby the collision safety is improved.
Moreover, in recent years, considering the repairability after collision in addition to the collision safety, a vehicle type, in which an exchangeable crash energy absorbing part such as a crash box is used for absorbing the shock, has been increased. This crash energy absorbing part may be mounted at a front surface and a rear surface of a cabin so that the shock-absorbing direction of the crash energy absorbing part is in a longitudinal direction of an automobile. The crash energy absorbing part is collapsingly deformed into a bellows shape in the shock-absorbing direction in collision and thereby absorbs the crash energy. Although it slightly differs depending on the vehicle type, there is a limitation in the shape of the crash energy absorbing part due to the space where the crash energy absorbing part is to be arranged.
Here, as shown by the views (A) to (E) in FIG. 1, the collapsing deformation into the bellows shape is performed by repeating deformation such that buckling creases bw, which are formed at a certain buckling wavelength H, are folded. Other than this deformation, there are cases in which the entirety of a part is bent, whereby deformation occurs unstably. In such deformation, the crash energy is difficult to absorb sufficiently.
Furthermore, a collision of an automobile does not necessarily occur in a direction parallel to the shock-absorbing direction of the crash energy absorbing part. Therefore, the crash energy must be absorbed even when a crash load is applied in a direction crossing the shock-absorbing direction (for example, a direction that is oblique to the shock-absorbing direction by an intersection angle of 10 degrees).
Accordingly, a crash energy absorbing part is required to be made so that the collapsing deformation into the bellows shape will occur reliably and stably regardless of the direction of a crash load that is applied, from the viewpoint of absorbing all crash energy, which is generated in a light collision (for example, a collision occurring at the speed of 15 km/hour), and thereby preventing damages to other members. In addition, it is very important to reduce the weight of a member from the viewpoint of improvement in fuel efficiency.
Techniques for strictly controlling the material and the shape parameters of a crash energy absorbing part have been developed heretofore in order to make the crash energy absorbing part so that the collapsing deformation into the bellows shape will occur more stably.
For example, according to Non-Patent Literature 1, the behavior of collapsing of a thin cylindrical member which receives a compressive load in an axial direction is controlled by a ratio σy/E, in which σy represents yield stress of a material and E represents a longitudinal elastic coefficient (Young's modulus). In this case, when the ratio σy/E is small, an axial symmetric buckling mode tends to occur, and when the ratio σy/E is great, an axial asymmetric buckling mode tends to occur.
Also, according to Non-Patent Literature 2, regarding the behavior of collapsing of a thin cylindrical member, the collapsing mode is changed by a ratio d/t of a diameter “d” of the member and the thickness “t” of the member.
On the other hand, Patent Literature 1 discloses a technique for configuring a crash energy absorbing part to be collapsingly deformed into a bellows shape. In this case, the crash energy absorbing part has a cross section of a polygon shape of a rectangle or more, and a ratio t/M of the thickness “t” and a circumferential length M of the cross section is controlled to be not less than 0.0025.
Patent Literature 2 also discloses a technique for configuring a crash energy absorbing part to be collapsingly deformed into a bellows shape. In this case, the crash energy absorbing part has a polygonal cross section, and a ratio of lengths of adjacent sides among the sides of the polygon of the cross section is controlled to be not greater than 2.3.
The above techniques of strictly controlling the material and the shape parameters of the crash energy absorbing part are findings that are effective for configuring a crash energy absorbing part, which is made of an ordinary metal material, to be collapsingly deformed into a bellows shape. However, in the case of a crash energy absorbing part that is constructed of a sandwich metal sheet, in which a surface layer that is formed of a metal sheet is laminated on each side surface of a core layer and is bonded together, it is difficult to provide a crash energy absorbing part by fully utilizing the following characteristics of the sandwich metal sheet, only by controlling the material and the shape parameters as described above. That is, the sandwich metal sheet is light in weight compared to a metal sheet and can be deformed at a short buckling wavelength.
It is reported that a crash energy absorbing part that is constructed of a sandwich metal sheet is collapsingly deformed into a bellows shape at a short buckling wavelength by controlling a ratio of the Young's modulus of the metal sheet of a surface layer and the Young's modulus of a core layer. The mechanism of this deformation is described below.
Since the core layer restricts the metal sheet on each surface of the core layer by bonding, the sandwich metal sheet can be modeled by two metal sheets 12 which are restricted relative to each other by elastic springs 11 (the view (A) in FIG. 2). Although a degree of freedom of deformation of the metal sheet 12 is different, the collapsing deformation mode of each of the two metal sheets 12 is equivalent to the collapsing deformation mode of a metal sheet 12 on an elastic floor 13 (the view (B) in FIG. 2). The elastic floor 13 corresponds to restricting elastic springs. Both of the two metal sheets 12 (the view (A) in FIG. 2) that are restricted by the elastic springs 11 are unfixed, whereas only the metal sheet 12 (the view (B) in FIG. 2) on the elastic floor 13 is unfixed. Therefore, the deformation of the elastic springs 11 corresponds to shear deformation in the case of collapsingly deforming the two metal sheets 12 that are restricted by the elastic springs 11, and the deformation of the elastic springs 11 corresponds to elongation deformation in the case of collapsingly deforming the metal sheet 12 on the elastic floor 13. Nevertheless, the collapsing energy is absorbed by the deformation of the elastic body and the deformation of the metal sheet in each of the cases. In this case, the deformation is performed so that the total of the deformation energy will be the minimum. When the metal sheet 12 of the surface layer is deformed at a buckling wavelength H1 (the view (C) in FIG. 2), which is equal to the length of the straight portion of the metal sheet 12, an energy er is the minimum. On the other hand, in the deformation of the elastic floor, the energy can be made smaller when the elongation is made as small as possible. Thus, when the metal sheet 12 is deformed at a short buckling wavelength H2 as shown in the view (D) in FIG. 2, an energy ec is the minimum. Accordingly, the buckling wavelength of the sheet on the elastic floor depends on the balance of the amount of the energy ec and ef and is thereby a value which is smaller than the bucking wavelength H1 and is greater than the buckling wavelength H2 (the views (C) and (D) in FIG. 2).
The sandwich metal sheet is collapsingly deformed at a short buckling wavelength by the same principle as in the case in FIG. 2. That is, in the surface layer, the deformation energy is small when the surface layer is deformed at a long buckling wavelength, whereas in the core layer, the deformation energy is small when the core layer is deformed at a short buckling wavelength. The sandwich metal sheet is deformed at a buckling wavelength, at which the amount of the deformation energy of the surface layer and the core layer is balanced and the total of the deformation energy of the surface layer and the core layer will be minimum. Since the core layer is deformed at a short buckling wavelength because the deformation energy is decreased, a crash energy absorbing part that is constructed of the sandwich metal sheet is collapsingly deformed at a shorter wavelength compared to a crash energy absorbing part that is made of a single material. However, in a sandwich metal sheet, in which a core layer has a high Young's modulus, and in which a hardly deformable material such as a brazing material is used as a bonding material, the core layer is hardly deformed and is difficult to deform at a short buckling wavelength. Therefore, in such a crash energy absorbing part, the collapsing deformation into the bellows shape may not occur stably.
In another example, Patent Literature 3 discloses a crash energy absorbing part which has a polygonal closed cross section with an inwardly recess portion, and in which a bending moment is differentiated at a part of the cross section. By forming such a complicated cross sectional shape, the buckling wavelength is made short, the collapsing deformation into the bellows shape stably occurs even in a collision from an oblique direction, and the crash energy is absorbed sufficiently. However, this technique can be used in the case of using a metal sheet. Therefore, if a sandwich metal sheet is formed into the same complicated shape as in the above technique, there is a high probability that a forming defect such as rupture of a surface layer occurs in the forming and a desired shape is not obtained.
As described above, in general, the material and the shape parameters of a crash energy absorbing part are controlled so that the crash energy absorbing part will be collapsingly deformed into a bellows shape even when an impact is applied in a direction crossing the shock-absorbing direction of the crash energy absorbing part. However, a technique for improving the fuel efficiency of a transportation vehicle and for obtaining sufficient absorbable amount of the crash energy by forming a crash energy absorbing part with a light weight material and making collapsing deformation into a bellows shape occur more stably, has not yet been developed.